Continuous junction growth



P. c. GOUNDRY ET AL 3,470,039

CONTINUOUS JUNCTION GROWTH Sept. 30, 1969 2 Sheets-Sheet l 7 Filed D60 21. 1966 mamx m IN\/;ENITOR James CBoatman Paul Cjoundry BY ATTORNEY Sept. 30, 1969 P. c. GOUNDRY ET AL 3,470,

CONTINUOUS JUNCTION GROWTH Filed Dec. 21, 1966 2 Sheets-Sheet 2 P--TYPE' N-TYPE United States Patent 3,470,039 CONTINUOUS JUNCTION GROWTH Paul C. Goundry and James C. Boatman, Richardson, Tex., assignors to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Dec. 21, 1966, Ser. No. 603,625 Int. Cl. H011 7/46 US. Cl. 148-172 Claims This invention relates to semiconductor material preparation and more particularly to a method of continuously joining semiconductor materials of different conductivities to produce continuous junction growth at the interface of the different materials.

The increasing demand for greater reliability of semiconductor devices and continued pressure for reduced costs necessitate extensive mechanization of semiconductor manufacturing processes. Furthermore, in preparing slices of single crystal material formed by the grown junction technique for fabrication of this material into semiconductor devices, the conventional operations such as sawing, lapping, polishing and etching, result in considerable waste of the original monocrystalline semiconductor material. A great need, therefore, exists for a method of continuous, direct growth from a melt of monocrystalline semiconductor material containing continuous junctions for use in semiconductor devices.

Therefore, an object of the invention is to provide a method for continuous, direct growth from a melt of monocrystalline semiconductor material containing continuous junctions for use in semiconductor devices. Still another object of the invention is to provide a direct, more rapid and less wasteful method than the practice of growing crystals of material containing grown junctions prepared by the introduction of dopants at a particular time during growth.

Other objects, features and advantages of the invention will become more readily understood from the following detailed description taken in conjunction with the appended claims and attached drawings in which:

FIGURE 1 is a diagrammatic representation partially in cross section of apparatus suitable for direct growth from a melt of monocrystalline semiconductor material containing a continuous P-N junction.

FIGURE 2 depicts a suitable guide for shaping the molten semiconductor material.

FIGURES 3 through 7 are cross sectional views of typical examples of the geometrical forms in which a P-N junction may be grown by the method of the invention.

It is to be understood that the figures of the drawings are not drawn to scale, the dimensions being exaggerated for purposes of illustration.

The method of the invention consists essentially of the use of a plurality of feed rods of semiconductor material of dissimilar conductivities, separated by a suitable partition or partitions to prevent mixture of the molten zones, and the use of a shaping guide containing a plurality of separate openings of suitable shape and size placed sufiiciently close to one another such that the shaped semiconductor material freezes above the shaping guides with the dissimilar materials meeting at or adjacent to the liquid-solid interface above the shaping guide.

Referring now to the drawings, FIGURE 1 depicts apparatus comprising a cylindrical chamber 1 made of quartz or other suitable material, having an inert gas atmosphere of for example argon, into which two feed rods 3 and 4 of semiconductor material such as silicon are continuously fed through openings in the bottom thereof. These openings are appropriately fitted with a gas seal 2. Feed rod 3 is a P-type semiconductor material and. feed rod 4 is an N-type semiconductor material. An RF heating coil 5 is disposed around the chamber 1 at the end of the feed rods to form molten masses 6 and 7 supported on the end of the rods by surface tension and by the separator 8. A shaping guide 9 is positioned over themolten zones 6 and 7. A focusing coil 10 provided with a pair of flow channels 11 and 12, is located beneath theheating coil 5. The region directly above the shaping guide 9 is liquid and the tip of the separator 8 marks the liquid-solid interface 13.

The maintenance of the liquid-solid interface 13 above the shaping guide and the separation of the P-type molten materials 6 and the N-type molten materials 7 until they have been drawn through the shaping guide openings and meet in the liquid-solid interface region are essential requirements for achieving the growth of a P-N junction as the material is drawn from the melt. The separator 8 for isolating the two molten feed zones may be quartz or other material suitable for use as a shaping guide, as described below and may extend above the surface of the shaping guide between the two shaping guide openings. However, eifective control of the two separate materials prior to their joining in the: liquid-solid interface region may be achieved without such extension of the separator and a small free space region may exist immediately above the surface of the shaping guide below the point at which the two liquids meet and solidify. The distance between the two shaping guide openings is dependent upon the dimensions of each opening and the surface tension of the materials being grown.

The growth of the P-N junction materials is initiated by the simultaneous introduction of two separate seeds 14 and 15 of identical orientation into the two shaping guide openings. The molten material is pushed into the shaping guide 9 which provides an isothermal section above the shaping guide holes which as essentially the same shape as the holes. Initiation of crystal growth is begun in a manner similar to conventional crystal growth, the seeds being dipped into the melt via the guide holes and withdrawn vertically to result in growth of monocrystalline semiconductor material containing a continuous P-N junction. Since the actual freezing of the molten semiconductor occurs above the shaping guide surface rather than in the shaping guide, high crystalline perfection is achieved.

The cross-section of the P-N junction growth by this technique is the intersection of the two geometrical forms established by the two shaping guide openings. Typical examples of such arrangements, in cross-section are shown by FIGURES 3-7. It is apparent that junctions between materials of different conductivities, as well as between materials of different conductivity types, may also be formed by the method of the invention. For example, junctions between N and N+, P and P+ and N or P and semi-insulating material may be formed. Thus a rod of semiconductor material containing a continuous junction can be grown and subsequently cut into slices for the fabrication of semiconductor devices, the cross-section of these slices being similar to those shown in FIG- URES 3-7. The geometry and location of the junction is fixed by the selected geometry and location of the shaping guide openings.

The guide 9 not only controls the shape of the liquid rods by providing an isothermal cross-section, but also exerts shaping force on the surface of the stock immediately below the apertures of the guide. Thus it is seen that the guide must be made of a material which is not wetted by the molten semiconductor, .must not contaminate the semiconductor melt, and must not nucleate spurious crystalline growth. Very pure quartz has been found to be a satisfactory shaping guide material for practically all semiconductor elements. Pyrolytic boron nitride is very durable and is an acceptable material for the guide, although under some conditions it may not be used with silicon.

The shape of the guide 9 and its heat capacity are important factors in determining the temperature of the liquid drawn therethrough. The guide should be shaped so that the temperature at the periphery of the apertures is uniformly maintained. Thus, the guide presents an isothermal profile which determines the temperature of the semiconductor material drawn therethrough.

In FIGURE 2, a guide of the preferred shape is shown. The bottom of the guide has a beveled area around each hole which aids in guiding liquid material from the top of the liquid semiconductor material into the holes.

The use of a rod or bar stock of feed material provides continuous operation. A suitable mechanical feed mechanism (not shown) can be provided to permit the successive addition of new rods as the initial rod is consumed without interrupting the growing process. This continuous feed process avoids the use of a melt contained in a crucible, thus eliminating a major source of contamination. Furthermore, conventional heating means such as an RF heating coil 5, is employed to provide a uniformly molten region.

Focusing coil 10 is mounted immediately below heating coil and is in the form of a hollow truncated cone having a center bore through which the chamber 1 extends. The focusing coil is made of highly conductive material such as copper, and is provided with a pair of flow channels 11 and 12 connected thereto, which supply a flow of cooling liquid to the focusing coil 10. Thiscoil is positioned within the normal thermal field produced by excitation of the RF coil. The focusing coil limits or modifies the shape of the thermal field produced by the RF coil 5 by absorbing power from the part of the thermal field. The shape of the focusing coil and its position relative to the heating coil 5 determines the shape and extent of the zone heated by induction. Thus the coil 10 is said to focus the RF field. By controlling the length of the molten zone, the focusing coil prevents the melt from falling ofi the top of the semiconductor rod.

Reflectors (not shown) may be used to provide optimum temperature control and permit proper location of the melt solid interface above each of the guide holes so that continuous c-rystal growth can be maintained. In the embodiment illustrated in FIGURE 1, a second quartz tube 20 fitting inside tube 10 serves to keep the shaping guide 5 in place, and tends to reduce heat loss due to radiation. Since the junctions are grown directly from the melt, many subsequent operations conventionally used in the diifused junction technique are eliminated. The method is direct, more rapid, and less wasteful of material than the practice of growing crystals of material containing grown junctions created by the introduction of dopants at a particular time during the growth.

It is to be understood that while the invention has been specifically described with respect to the growth of a single continuous junction, the method is equally applicable to the growth of two or more junctions by the addition of one or more feed rods of semiconductor material of dissimilar conductivity, separated by a suitable partition or partitions to prevent mixture of the molten zones and by the use of a shaping guide having the appropriate number of separate openings of suitable shape and location.

Although the invention has been described with refer- 4 ence to a preferred embodiment thereof, it is to be understood that the forms of the apparatus by which the invention is practiced are to be taken as the preefrred embodiments of the same, and that various changes may be resorted to without departing from the spirit and scope of the invention as defined by the appended claims.

What is claimed is:

1. In the process of forming a monocrystalline rod of semiconductor material, the steps of:

(a) heating the end of a feed rod of semiconductor material having a first conductivity to form a molten mass thereof supported on the end of the rod by the surface tension of the molten mass,

(b) heating the end of the feed rod of semiconductor material having a second conductivity to form a molten mass thereof supported on the end of the rod by the surface tension of the molten mass,

(c) maintaining the first molten mass separate from the second molten mass until they have passed through spaced shaping guide apertures, and

((1) drawing the molten masses through the spaced apertures so that the two molten masses are drawn together at the liquid-solid interface by virtue of their molten condition to form a continuous junction at the interface between the once molten masses.

2. In the process of forming a monocrystalline rod of semiconductor material, the steps of:

(a) heating the end of a feed rod of semiconductor material of a first conductivity type to form a molten mass thereof supported on the end of the rod by the surface tension of the molten mass,

(b) heating the end of a feed rod of semiconductor material of an opposite conductivity type to form a molten mass thereof supported on the end of the rod by the surface tension of the molten mass,

(c) maintaining the first conductivity type molten mass separate from the opposite conductivity type molten mass until they have passed through spaced shaping guide apertures, and

(d) drawing the molten masses through the spaced apertures so that the two molten masses are drawn together at the liquid-solid interface by virtue of their molten condition to form a continuous P-N junction at the interface between the once molten masses.

3. The method according to claim 4 wherein the material having a first conductivity type is P-type and the material having an opposite conductivity type is N-type.

4. The method according to claim 2 wherein said semiconductor material is silicon.

5. The method according to claim 2 wherein said shaping guide is quartz.

References Cited UNITED STATES PATENTS 2,743,200 4/1956 Hannay 148-115 3,124,489 3/1964 Vogez et al. 23-301 XR 3,198,671 8/1965 Dikholf 23-301 XR 3,261,722 7/1966 Kozler et al. 148-116 L. DEWAYNE RUTLEDGE, Primary Examiner P. WEINSTEIN, Assistant Examiner US. Cl. X.R. 

1. IN THE PROCESS OF FORMING A MONOCRYSTALLINE ROD OF SEMICONDUCTOR MATERIAL, THE STEPS OF: (A) HEATING THE END OF A FEED ROD OF SEMICONDUCTOR MATERIAL HAVING A FIRST CONDUCTIVITY TO FORM A MOLTEN MASS THEREOF SUPPORTED ON THE END OF THE ROD BY THE SURFACE TENSION OF THE MOLTEN MASS, (B) HEATING THE END OF THE FEED ROD OF SEMICONDUCTOR MATERIAL HAVING A SECOND CONDUCTIVITY TO FORM A MOLTEN MASS THEREOF SUPPORTED ON THE EDN OF THE ROD BY THE SURFACE TENSION OF THE MOLTEN MASS, (C) MAINTAINING THE FIRST MOLTEN MASS SEPARATE FROM THE SECOND MOLTEN MASS UNTIL THEY HAVE PASSED THROUGH SPACED SHAPING GUIDE APERTURES, AND (D) DRAWING THE MOLTEN MASSES THROUGH THE SPACED APERTURES SO THAT THE TWO MOLTEN MASSES ARE DRAWN TOGETHER AT THE LIQUID-SOLID INTERFACE BY VIRTUE OF THEIR MOLTEN CONDITION TO FORM A CONTINUOUS JUNCTION AT THE INTERFACE BETWEEN THE ONCE MOLTEN MASSES. 